The VITRO (Vitamins and Thrombosis) study investigated the effect of homocysteine lowering by daily supplementation of B-vitamins on the risk reduction of deep-vein thrombosis and pulmonary embolism. Patients between 20 to 80 years old with a first objectively confirmed proximal deep-vein thrombosis or pulmonary embolism in the absence of major risk factors and a homocysteine concentration above the 75th percentile of a reference group were asked to participate (hyperhomocysteinemic group). A similar study was conducted in a random sample of patients with a homocysteine below the 75th percentile of the reference group (normohomocysteinemic group). After informed consent patients were randomized to daily multivitamin supplementation (5 mg folic acid, 50 mg pyridoxine and 0.4 mg cyanocobalamin) or placebo and were followed for 2.5 years. End-points were objectively diagnosed recurrent deep-vein thrombosis or pulmonary embolism.